2,6-Diisopropylphenol, generically named propofol, is a well-known and widely-used, injectable anesthetic with hypnotic properties used both as a sedative, and to induce and maintain general anesthesia. It is sold as Diprivan (trademark Zeneca) for human use and Rapinovet (trademark Zeneca) for veterinary use. Propofol is administered directly into the bloodstream either by bolus injection or by infusion. Because the onset of anesthesia is largely controlled by a drug's diffusion rate through the blood-brain barrier, propofol's lipophilicity is key to its rapid activity. This lipophilicity, however, renders propofol relatively insoluble in water, hence it must be administered in conjunction with solubilizing agents, surfactants, or solvents; or as oil-in-water emulsions (Jones et al. (1998) U.S. Pat. No. 5,714,520). All references cited herein are incorporated by reference in their entirety.
As a parenterally administered agent, sterility of propofol formulations is essential. Commercial formulations are oil-in-water emulsions containing approximately 1% -2% propofol in 10% soybean oil. These formulations also typically contain a surfactant, 1.2% egg phosphatide for example, 2.25% glycerol to make the formulation isotonic, sodium hydroxide to adjust the pH to physiological pH, and 0.005% EDTA equivalent (as edetate) to retard microbiological growth (all weights approximate) (Id.). Edetate containing formulations are not antimicrobially preserved by USP standards; however, microbial growth is retarded (Id.).
Non-preserved, propofol oil-in-water emulsion formulations have significant drawbacks arising from the fact that these formulations support microbial growth: strict aseptic handling technique is required; maximum utility time is 12 h maximum after vial entry. Handling recommendations include immediate administration after vial entry, and disposal of infusion assemblies and of unused material after 12 h. Nevertheless, reports of nosocomial infections resulting from adventitious contamination are not uncommon (Bennett et al. (1995) N. Engl. J. Med. 333:147-154). Improper handling techniques include delayed administration after transfer from vial to syringe, use of 50-and 100-mL products as multi-use, for multiple patients, and use of 50 and 100 mL products for an extended time period.
An application for which preserved propofol formulations are particularly advantageous is their use as a long-term sedative by continuous infusion. The risk of microbial contamination of non-preserved propofol in infusion devices increases both with residency-time in the infusion device, and with increased manipulation of the device. The utility time of formulations containing EDTA salts (edetates) is at least 24 h compared to 6 to 12 h for non-preserved formulations (Jones et al.). A longer lasting formulation means that fewer manipulations are required. The consequent reduced manipulation accrues a number of important benefits: reduced probability of microbial contamination, reduced probability of operator error, reduced drug waste, and reduced labor intensiveness--all of which combine to increase safety and reduce costs.